Pelagic Cretaceous sediments, deposited in a range of palaeotectonic and palaeogeographic settings, from continents to oceans, are commonly black and bituminous. 3 particular time-envelopes define the major occurrences of such facies: late Barremian-Aptian-Albian, the Cenomanian-Turonian boundary and, to a lesser extent, the Coniacian-Santonian. These intervals define the duration of so-called Oceanic Anoxic Events during which global marine waters were relatively depleted in oxygen, and deposition of organic matter, derived from both terrestrial and planktonic sources, was widespread. Cretaceous OAEs correlate closely with transgressions, and such a correlation exists throughout the stratigraphical column. Flooding of land-masses is thought to have transported much terrestrial plant material seawards; the progressive increase in shelf-sea area is thought to have stimulated production of marine plankton. Bacterial consumption of this organic matter favoured the development of poorly oxygenated mid-to late Cretaceous waters in which many of the characteristic facies of the Period, including glauconitic sandstones and phosphatic chalks, were deposited.

Cretaceous anoxic events: from continents to oceans

The phosphorus cycle, phosphogenesis and marine phosphate-rich deposits

Phosphorus is an essential nutrient for all life-forms It is a key player in fundamental biochemical reactions involving genetic material (DNA and RNA) and energy transfer (ATP) and in structural support of organisms provided by membranes (phospholipids) and bone (the biomineral hydroxyapatite) Photosynthetic organisms utilize dissolved phosphorus, carbon, and other essential nutrients to build their tissues using energy from the sun Biological productivity is contingent upon the availability of phosphorus to these simple organisms that constitute the base of the food web in both terrestrial and aquatic systems It begins with a brief overview of the various components of the global phosphorus cycle Estimates of the mass of important phosphorus reservoirs, transport rates (fluxes) between reservoirs Following the overview, various aspects of the global phosphorus cycle are examined in more depth, including a discussion of the most pressing research questions currently being posed and research efforts presently underway to address these questions

The Global Phosphorus Cycle

Chapter 33 – The Mineralogy and Geochemistry of Near-shore Sediments

The modern sediments accumulating on the Namibian shelf between 20° and 26°S latitude are composed predominantly of biogenous material: organic matter, diatomaceous silica and calcium carbonate. A belt of anoxic, organic-rich diatom ooze, containing fish debris and Recent and pre-Recent phosphorite, occurs in a coastal zone up to 70 km wide off Walvis Bay in water depths shallower than 140 m. A blanket of foraminiferal and skeletal calcarenites, containing reworked phosphorite, and calcareous muds covers the central and outer shelf. The diatom ooze is a modern equilibrium facies reflecting the intensity of coastal upwelling in the Benguela Current; sedimentation rates range between 30 and 120 cm/103 yr. Seaward of the diatom ooze, benthic and planktonic carbonate is mixed with a reworked, shelly transgressive facies on the middle part of the shelf, the sediment becoming richer in foraminifera and more fine-grained towards the shelf edge, where sedimentation rates are on the order of 5 cm/103 years.

K.I. Merkulova

Papers

Radiometric evidence for recent formation of phosphatic nodules in marine shelf sediments